RESPIRATORY
SYSTEMThe
gills lie in the two branchial chambers formed by the branchiostegites
of the carapace.Freshwater
crabs typically have nine pairs of gills (the same as marine crabs).

In aquatic
freshwater crabs, the
majority of the respiratory water enters the gill chambers ventrally
through the inhalant openings (the 'Milne-Edwards openings') situated
between the basal joints of the chelipeds between the basal joints
of the walking legs and the neighbouring margin of the carapace.
Exhalent water leaves the gill chambers via the two efferent canals
on either side of the buccal cavity. The respiratory stream is
maintained by the beating action of the gill bailers (scaphognathites)
which are part of the second maxillae.

A reduction in the number
of gills is seen in some air-breathing species
such as the terrestrial African freshwater crab Globonautes
macropus. Terrestrial species
of African freshwater crabs (e.g., Globonautes macropus)
regularly carry out aerial respiration by means of a "lung"
(called a "pseudolung") that consists of a fleshy vascularized
membrane in the dorsal part of the gill chamber, although this
species has also retained fully functional gills in the ventral
part of the gill chamber.

Photograph
of the pseudolung of Globonautes macropus from Liberia.
Note the
perforated appearance and fleshy structure of this respiratory
organ.

REPRODUCTIVE SYSTEMMale. The reproductive system of male freshwater crabs
consists of a pair of testes, a pair of vas deferens (ducts between
the testes and the penes), two penises and two gonopods. The testes
of male African freshwater crabs lie in the cephalothorax on top
of the hepatopancreas. They produce spermatozoa that are carried
away in paired ducts (the vasa deferens) which open ventrally.
The right and left penises are short, soft, flexible membranous
tubes located at the terminal ends of the vasa deferens. The penises
are sited on the coxae of pereiopods 5 and do not pass through
the sternum in any species of freshwater crab.

Gonopods
of Potamonautes dybowskii in natural resting position beneath
the abdomen

Copulation.
Gonopods 1 and 2 are paired abdominal appendages (pleopods) that
are modified to function as copulatory organs that transfer the
spermatophores from the male penises to the female sexual openings.
The gonopod 1 of freshwater crabs is a three or four-part hollow
tubular organ with an apical opening in the terminal part. The
long thin, broad-based gonopod 2 fits tightly into the subterminal
segment of gonopod 1 and leaves a lateral basal opening into which
the penis extends. Male and female freshwater crabs copulate in
the normal brachyuran way, by laying head-to-head and sternum-to-sternum,
with their abdomens relaxed so that the abdomen of the female
overlaps that of the male. This brings the female openings into
contact with the gonopods which swing out away from the sternum
when the male abdomen is relaxed. The terminal articles of gonopods
1 connect with, and are inserted into, the paired vulvae of the
female sited on sternite 5. The spermatozoa (in spermatophores),
together with the secretions of the vas deferens, are ejected
through the penis into the subterminal gonopod chamber between
the bases of the subterminal segments of the two gonopods. There
is only a single spermatozoa in each spermatophore (i.e., freshwater
crabs exhibit cleistospermy). During copulation the pumping action
of gonopod 2 forces the spermatophores up the gonopod chamber
inside the two gonopods. The spermatophores are pumped out of
the apical opening of the terminal article of gonopod 1 into the
spermathecae that lie just deep to the female sexual openings,
where they are stored until the eggs are laid.

Female. The reproductive system of female freshwater crabs
consists of the paired ovaries, ovarian ducts, spermathecae, vaginas
and vulvae. The ovaries of female African freshwater crabs lie
in the cephalothorax on top of the hepatopancreas. The eggs are
carried away in paired ovarian ducts that open ventrally through
the pair of female openings (vulvae) on sternite 5. The vulvae
are elongated sideways to receive the first gonopods, and the
distal parts of the oviducts just deep near to the openings have
spermathecae to receive and store the spermatophores. Female crabs
store the spermatophores for long periods, and the eggs do not
have to be laid immediately after mating. Sperm in the spermatophores
fertilises the eggs as they are laid. The fertilised eggs are
attached to the female abdominal pleopods by long sticky threads
secreted by the female.

Development. True freshwater crabs spend their entire lives
in fresh water and it is a shared defining characteristic to be
able to complete their entire life cycle independently of sea
water. All true freshwater crabs lay their eggs and rear their
young in a freshwater environment, rather than in a brackish or
marine habitat like other crabs. The adaptation of true freshwater
crabs to freshwater environments has involved a number of modifications
of their reproductive system and behaviour. These modifications
include the production of relatively few eggs, each of a relatively
large diameter, a complete reduction of the larval stages ("direct
development") whereby the eggs hatch directly into juvenile
crabs ("hatchlings"), and the protection of the hatchlings
for several weeks after egg hatching. The number of eggs laid
by species of marine crabs varies with the size of the species,
and large-bodied species can lay from several thousand to more
than a million eggs. In contrast, female freshwater crabs produce
far fewer eggs: by the hundreds, rather than by the thousands
or tens of thousands. The newly laid eggs of marine crabs are
very small (0.25-0.35 mm in diameter), and double in size as they
develop. In contrast, the newly laid eggs of freshwater crabs
are much larger (about 1 mm in diameter), increase to between
3 and 5 mm in diameter as they develop, and remain attached to
the pleopods of the female until hatching. The newly laid eggs
of African freshwater crabs are bright orange and change colour
slowly to dull brown, dirty grey, and then to black before they
finally hatch into small crabs. Young freshwater crabs leave the
eggs as small versions of adult crabs rather than as larvae. The
hatchlings are retained on the females pleopods in the females
abdominal brood pouch for several weeks after hatching and female
freshwater crabs show a degree of maternal care.

Freshwater
crabs were originally known as river crabs, and were given family
names such as Thelphusidae and Potamonidae, the latter name being
derived from the Greek word root Potamon meaning river.
While the river-living habit is an accurate description of the
lifestyle of some species of freshwater crabs it by no means applies
to all species. In Africa, as in other parts of the tropics, some
species of freshwater crabs have moved out of rivers, streams
and lakes and have colonised nearby land. In forest and savanna
ecosystems, the move onto land favoured (among other things) the
development of the ability to breathe air as well as water. Some
species of freshwater crabs rival land crabs (Gecarcinidae) in
their degree of independence from water.

Other
Crabs Found in Freshwater Habitats

Other
crabs found in tropical freshwaters may not be true freshwater
crabs, but actually may belong to a marine-breeding family that
can tolerate dilute conditions for part of its life but
which needs seawater at some point in its life cycle. Examples
include members of the Sesarmidae, Varunidae, Hymenosomatidae,
Portunidae and Xanthidae.